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Fiber Bragg gratings (FBGs) are important optical elements widely used in various functional devices for dense wavelength-division multiplexing (DWDM) networks, for example FBG-stabilized laser sources and various FBG-based WDMF devices for multiplexers, demultiplexers, and add/drop filters. In these applications of FBGs, a problem arising from changes in the ambient temperature has been observed. Because the spacing of the Bragg grating determines the central wavelength of a reflected optical signal in an optical fiber, have to be carefully designed and accurately manufactured. As with most optical devices, the reflected central wavelength of an FBG varies with the ambient temperature of the device. This is caused only partially by expansion and contraction of the fiber with temperature and consequent change in the period of the refractive index variations in the fiber core. The dominant effect is a variation of the refractive index itself with temperature. These variations, which can be as small as 50 GHz (0.4 nmn), are nevertheless undesirable in view of the narrow channel spacings used in high-performance optical communication systems. Thus, reducing the thermal variability of the FBGs is a key factor to their commercial success in the telecommunication industry.
FBGs can be fabricated by interferometric or phase-mask techniques. However, their packaging is a vital technology in making FBGs suitable for real-world applications. Annealing, laser welding, epoxying, and recoating during fabrication can result in deviation from the desired central wavelength. Thus a packaging device designed with a posttuning mechanism is necessary. To compensate the FBGs thermal wavelength shift, that mechanism must provide both positive and negative correction. One of the methods to achieve this is to include a prestressed element (stretcher) in the packaging device (see S. I. Lin, xe2x80x9cTemperature-compensating device with central-wavelength tuning for optical fiber gratings,xe2x80x9d Opt. Eng. 40(5), 698-702, May 2001). Although stretching a fiber can be done with practically anything, for example a small motorized translation stage, typically more precise stretching is required.
Mechanical strain (stretching) in the fiber also changes the reflected central wavelength of the FBG. The design of stretchers is based on the effects of both strain and temperature on wavelength shift and hence a dispersion shift. The use of strain (stretch) is also the mechanism for tuning dispersion in FBG devices. One effect is used to provide a proportional offset for the other to ideally produce a net zero wavelength shift across a given temperature range. To calculate this proportional offset, the relevant constants are temperature effects on wavelength shift of an unsupported fiber xcex94xcex(T)=+0.01 nm/xc2x0 C.; coefficient of thermal expansion for fused silica =+0.5 xcexcxcex5/xc2x0 C.; and strain effect on wavelength shift xcex94xcex(xcexcxcex5)=+0.001 nm /xcexcxcex5, where the symbol (xcexcxcex5) represents parts per million.
Accordingly, to thermally compensate the fiber, a strain of ≈xe2x88x929.5 xcexcxcex5/xc2x0 C. must be applied to the fiber to produce a near zero wavelength shift. In other words the fiber must be relaxed 9.5 parts per million for every degree C increase of temperature. If a device is constructed that will put tension onto the FBG at low temperatures and relax that tension progressively as the temperature is increased, the two temperature dependent effects can compensate one another. This is commonly done by packaging the fiber with another material that has different thermal expansion characteristics from that of fiber. FIG. 1A is a schematic cross-section depicting a fiber 12 installed in a prior art thermally compensated stretcher 10 (see H. Dutton, xe2x80x9cUnderstanding Optical Communications,xe2x80x9d Prentice Hall, 1998, 275-276). Fiber 12 including a FBG segment 13 is centered inside a hollow fused silica tube 14. Fiber 12 is bonded to reentrant steel end caps 15 typically using epoxy 16. Steel has a much higher coefficient of thermal expansion than the silica of tube 14 or of fiber 12 itself. Fiber 12 is bonded into steel end caps 15 under tension at the lowest temperature at which the device operates. When the temperature rises, the differential expansion of steel end caps 15 causes the tension in fiber 12 to relax. This variation in tension is arranged to cause wavelength variations in the opposite direction from those caused by normal thermal movement of the unsupported fiber. Both effects balance to produce a substantially stable device without active temperature control. However, the simple stretcher of FIG. 1A has no fine adjustment capability and provides coarse adjustment over only a limited range of tension.
The sensitivity of the reflective spectrum center wavelength of intracore Bragg gratings to the strain and thermal environment to which they are subjected has made them popular as sensors and as tuning elements for fiber and diode lasers. Chirped gratings have been shown to compensate for dispersion experienced by short optical pulses traversing a length of optical fiber. A number of methods of fabricating chirped gratings have been devised, but each grating is limited to a fixed narrow range of dispersion compensation.
Sources of optical fibers with FBGs include the following: TeraXion Inc., 20-360 rue Franquet, Sainte-Foy, Quebec, G1P 4N3, Canada; Southampton Photonics (US) Corporate Office, 170 Knowles Drive, Suite 2, Los Gatos, Calif. 95032; and Redfern Photonics Pty Ltd Headquarters, Suite 212, National Innovation Centre, Australian Technology Park, Eveleigh, NSW, 1430, Sydney, Australia.
FIG. 1B is a schematic diagram depicting a prior art stretcher 11 in which a Bragg grating attached to a structural element is strained nonuniformly by means of a set of linear actuator elements (see U.S. Pat. No. 5,694,501, issued Dec. 2, 1997 to Alavie et al.). Fiber 12 is coupled to a segmented piezoelectric stack 17 that can be selectively energized at a number of electrodes 18 along the length of the grating. Strain effects can be fine tuned using voltage control 19. However, piezoelectric stacks are capable of inducing strain over only a limited range, and this arrangement can cause strain discontinuities at the interfaces of the piezo elements that create the stack, causing a detrimental effect on the optical signals being compensated.
The present invention is directed to a system and method which is directed to thermal compensation in a fiber stretcher.
Embodiments of the present invention provide technical advantages not available in the prior art. Temperature compensated fiber stretching is accomplished by amplifying with a unique flexure the relative displacement between a rigid frame and a longitudinal spacing element, both in contact with the flexure. For passive thermal compensation, the relative displacement is provided by differential thermal expansion between the rigid frame and the longitudinal spacing element. For active thermal compensation and/or for wavelength tuning when the fiber contains an embedded fiber Bragg grating, the relative displacement is provided by a combination of differential thermal expansion and active feedback control of the length of the longitudinal spacing element, which is a linear actuator. The feedback control can be configured to stabilize actuator length using length feedback from a displacement sensor and/or to actively control length using feedback from a temperature sensor. The feedback loop can include one or more logic elements, for example algorithms or look up tables. In an optical system, combined passive compensation and active control with flexure amplification provide superior thermal wavelength stability and tuning range.
Embodiments of the present invention are compact, rugged, and easily manufacturable. In some embodiments, the rigid frame and the flexure are fabricated from the same material. In further variations, the rigid frame and the flexure are portions of a single monolithic piece of material.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.